ARTIFICIAL FLIGHT. - 541 



engine weighing eleven pounds per horse-power is a differ- 

 ent matter to the construction of a one-horse-power engine 

 weighing eleven pounds ; and as the primary object of all 

 experiments is to obtain a machine that will carry the 

 weight of a man, there are certain limits of size below which 

 machines would not be of practical use. It may be noted, 

 too, that the steam turbine, owing to its absence of vibra- 

 tion, promises to be a very useful and suitable motor for the 

 purposes now considered, and may in all likelihood super- 

 sede engines with reciprocating parts. 



(ii.) Propellers and aeroplanes. — Flight like all other 

 forms of locomotion depends on the principle of action and 

 reaction, but there is this important difference, that whereas 

 in a rowing boat the reaction of the water on the blades of 

 the oar only has to move the boat forward, in a bird or 

 flying machine the reaction of the air set in motion by the 

 wings or aeroplane has to continually overcome gravity. 

 To keep a body heavier than air afloat in the atmosphere 

 it is necessary that downward momentum should be con- 

 tinually communicated to the surrounding air at a rate 

 proportional to and measured by the weight of the body. 

 This communication of momentum involves the continuous 

 expenditure of work, but this work is measured by the 

 kinetic energy and not the momentum imparted to the air. 

 This kinetic energy for given momentum is inversely pro- 

 portional to the mass of air acted on, hence in order that 

 there may be as little waste of power as possible in sup- 

 porting an aerial machine it must be provided with 

 aeroplanes acting on as large a mass of air as possible. 

 Now there are two ways of acting on a great mass of air ; 

 one is by constructing enormously large aeroplanes, the other 

 is by imparting a considerable horizontal motion to the planes. 

 For if we move a plane rapidly through the air in a hori- 

 zontal direction it will constantly come in contact with fresh 

 particles of air in succession, and it will thus be supported by 

 a far larger mass of air than it would if it were to constantly 

 remain in the same place, simply on account of the rate at 

 which it moves. This property is not unknown to engineers; 

 the ordinary screw propeller of a ship affords another illus- 



